During printing, ink is deposited on the surface of a substrate, made, for example, of paper or plastic. It is then common to cover the printed surface of the substrate with a protective layer. This protective layer completes the fixation of the printed image on the substrate while protecting the print against certain external aggressions such as projections, and/or light, heat and humidity. The protective layer is typically formed of an ink like material that enables creation of different visual effects by printing the ink like material as patterns on certain areas. Deposition of this protective layer on a printed substrate is generally performed by flexographic, offset, screen printing. It is also possible to customize the protective layer by causing some areas of the substrate to have varnish patterns and other areas to not have any varnish.
Patent application EP 1749670 relates to a digital ink jet machine for laying a coating with medium viscosity on a variable coating substrate by using ink jet nozzles including hollow needles set into vibration by a piezoelectric actuator adhesively bonded to a resonator including an assembly of the hollow needles. The size and the shape of a droplet of material (i.e., an ink or varnish) deposited by each nozzle on the surface of the substrate, depend on the duration and amplitude of an electric wave driving the actuator. However, such a device, if it is able to deposit varnish drops with medium or high viscosities (which are of the order of about a thousand centipoises), is not suitable for optimum ejection of varnishes with much lower viscosity.
A goal of the present invention is to overcome one or more drawbacks of the prior art by providing a suitable device for optimal deposition of a protective ink layer on a surface of a substrate independently of the ink viscosity and the substrate to be covered.